Tanjiro getting mad when somebody calls Nezuko ugly may be a subconscious desire to prove to others that despite being a demon, Nezuko is still the beautiful and kind young woman she once was.
In Season 1 Episode 19 Tanjirou's blade starts glowing bright red when he uses Hinokami Kagura for the first time. At first it looks like a visual effect for his new move, but later on in the manga, its revealed bright red nichirin blades disrupt a demon's regenerative abilities, a move that the Hashira and Tanjirou eventually use against Muzan.
If Zenitsu is so scared of the danger that fighting against demons in the frontlines entails, then why does he remain a demon slayer at all? The series shows that there are positions within the Corps that don't involve direct combat, thus, in theory, it should be safe enough for a nervous wreck like him. This is foreshadowing one of the foundations of Zenitsu's true character. The guy is easily scared but he is also extremely loyal and grateful, his master poured so much attention and trust on him that Zenitsu doesn't dare to abandon his sword, even if his fear screams that he should just run away and leave it all behind.
There's a reason why Mist Breathing and Beast Breathing are branched from the Wind Breathing style. Wind Breathing is all about unorthodox fighting techniques. Sanemi is shown to use very unorthodox fighting techniques like doing a surprise spinning kick that luckily just grazed Tanjiro's ear during an argument in Hashira Training or sliding under Kokushibo's legs to open up for a sword technique and using Genya's sword with his feet to try a sneak attack under Kokushibo's chin. Mist Breathing derives from this by focusing on deception and intentionally confusing the opponent. Muichiro uses very skilled sword techniques and swift movements to deceive his opponents and also wears long clothes to conceal his body proportions according to the Databook. Beast Breathing is... well... Inosukebeinghimself, and he also has Abnormal Limb Rotation Range, which makes him very unpredictable.
Serpent Breathing derives from Water Breathing because Water Breathing is all about fluidity. Serpent Breathing adds its own spin on this by focusing on flexibility and swift sword-swinging movements that can go in unpredictable directions. Iguro can use his sword techniques to swing his sword in unusually shaped directions and it's especially effective with his movement speed.
Sound Breathing derives from Thunder Breathing because Thunder Breathing users have great speed and hearing. Sound Breathing focuses on the hearing aspect. Tengen uses the sounds and rhythm of movement to predict his opponent's next attacks and can find openings to use his own techniques. Although, Sound Breathing also utilizes speed since Tengen is shown to be even faster than Zenitsu.
Love Breathing derives from Flame Breathing because Flame Breathing uses techniques that grow stronger through every use. Love Breathing adds its own spin by focusing on the continuous use of the techniques with speed and power (DPS by videogame standards) and Mitsuri is perfectly capable of doing so because of her physically fit body.
The Mark causes all who manifests it to die at the age of 25. Yoriichi, however, was shown to have survived for a good eighty years before he died of old age. How did he survive for so long when everybody else died at twenty-five? As seen from his birth, it appears Yoriichi was born with the Mark. Meaning his body may have adjusted to whatever harmful effects it had early on in life.
In Kanao's side story, she is extremely withdrawn socially, emotionally, and often must receive orders to even do basic necessities. When Shinobu complained to her older sister Kanae about Kanao, Kanae remarks that Kanao will blossom when she finds a guy that she likes. The guy in question that shows up: Tanjiro Kamado. This reflects on their Breathing styles: Kanao's Flower Breathing, and Tanjiro's Water Breathing. Tanjiro's hanafuda earings and Hinokami Kagura even suggest that he is a living legacy to the Sun Breathing. What do flowers need to grow up beautifully? Water and Sunlight!
A fun fact too: Kanao's birthday (or rather the day where the Kocho sisters took her in) is on May 19, which makes her a Taurus (an earth sign) and the month is on spring, where flowers usually bloom. Tanjiro, meanwhile, is born on July 14, which makes him a Cancer (a water sign) but was born around summer (fire is the common element being associated to that said season).
Nezuko is able to regain a part of her humanity such as talking and sunlight immunity simply because she refuses to consume any human flesh or blood. The lesser extent can be applied to Tamayo as well, although it took her many years to snap out of her funk and regain her humanity, all she needs to replenish herself is a small amount of blood (which she buys from willing donors). Had Muzan been more kindhearted and understanding towards people like Nezuko and Tanjiro are, he would've gotten the immediate help needed to cure his disease and regain his humanity a long time ago. It was his own stubbornness and evil nature that caused him to end up like this, beginning when he murdered the kind doctor that tried to help him on impulse and losing the recipe for the original medicine in the process. Small wonder that Muzan prefers to transform humans who are crossing the Despair Event Horizon (e.g. Hakuji/Akaza, Gyuutaro and Rui) and humans who are plain Jerkass (e.g. Kaigaku) into demons, just to cause more trouble.
Then the final chapter highlights just how Muzan's cruelty denied him the blue spider lily he was seeking; the flower only grows for a few days in broad sunlight. So the only way Muzan could ever get the flower would be for a human to get it for him. Because he has no empathy for people, regardless of whether they're humans or demons, Muzan ultimately lost the easiest way to get the immortality he desired without ever knowing it. In the second fanbook, the irony is taken even further as it reveals that Kie and Tanjiro knew of an area near their house where the blue spider lily would sometimes grow. That's not even mentioning that the specific place happens to be where Yoriichi buried Uta and their baby.
Nezuko's resistance to human blood, particularly Sanemi's when flaunting his flesh around her, is even more impressive when one can recall the latter has the rare Marechi type with an intense potency for the smallest whiff immediately causing demons to salivate, lose focus and go crazy for it. All to further prove the Hashira and the Demon Slayer Corps that she is completely harmless.
The explanation of Muzan's backstory from chapter 127 explains that he needs a demon who can resist sunlight or the blue spider lily flower to gain his own sun immunity. So it's possible that his reason for making other demons is not only to create an army for himself but also to find a demon that can gain sun resistance for him to absorb. Nezuko walking in the sun is the perfect opportunity that Muzan has always longed for.
Volume 17's extra chapters further confirms that the elemental effects are not actually being produced by the Breathing users techniques, rather they're images created by the viewer (i.e. Tanjiro and Giyu don't actually create water from their swords because if they did, there would be signs of wet spots in the area). But, Muzan and his demons are able to use magic that can cause unnatural effects to happen (i.e. Kyogai being able to shift the gravity of his house domain by tapping his drums). So, if you really think about it, it's a literal battle between Badass Normal human warriors and demons with supernatural powers.
A volume extra comparing the Hashira's raw strengths through arm wrestling reveals Iguro is a bit weaker than Muichiro, who is 14. Though this makes sense; Muichiro was from a family of woodcutters and is shown in flashbacks helping his brother chop and carry wood, growing a good physical shape even before becoming a Demon Slayer. Iguro meanwhile spent twelve years of his life locked in a cage with poor ventilation, which negatively impacted his overall growth, including body strength.
Tanjiro using the Hinokami Kagura full-time during the Infinity Fortress arc is the result of Tanjiro's dedication in the Hashira Training arc. Since the Hinokami Kagura is actually the Breath of the Sun, it makes sense that Tanjiro gets tuckered out after each use prior to the Training Arc since as Yoriichi Tsugikuni demonstrated, the human body needs to be in a peak condition in order to fully utilize it's techniques. While Tanjiro knows the forms, he only trained in Water Breathing and thus his body cannot handle Sun Breathing which is the amalgamation of all the basic breathing forms. It isn't until Tanjiro undergoes the Hashira Training and takes in their strengths and disciplines that he is able to use the Hinokami Kagura in it's full glory.
The face-offs between the Twelve Kizuki and the Demon Slayers could not have been any more fitting.
Tanjiro has a severe grudge against Upper Moon 3 Akaza second only to Muzan himself, and Akaza reciprocates this vowing to slaughter Tanjiro "limb from limb" after the events of the Mugen Train Arc. It's all too clear they are destined to fight, and Giyu joining Tanjiro's side clearly shows the close relationship between the two since the very first episode. It makes sense why Giyu, especially considering the Character Development from Tanjiro's influence, would defend the kid who's been his Only Friend (besides Shinobu to some degree) throughout the current events of the series.
Shinobu, Kanao, and Inosuke all have a bad history with Upper Moon 2 Doma, making their matchup with him very personal. Doma is the one who killed Shinobu's sister, Kanae—so naturally, Shinobu hates his guts. But when Doma absorbs Shinobu just in time for Kanao to arrive, Kanao goes all Haganezuka on him, as he just killed another adoptive sister of Kanao without any concern or regret whatsoever. Then, when Inosuke joins the fight, Doma reveals a little too much of his history with the former's mother, being the one responsible for her death, to which Inosuke rightfully responds by promising to "create hell for him." No other demon can traumatize the aforementioned Demon Slayers like Doma could.
Upper Moon 1 Kokushibo, being the strongest Demon next to Muzan himself, is naturally pitted against three of the strongest Hashiras in the Demon Slayer Corps: Gyomei, Sanemi, and Muichiro, the last of which is his direct descendant. Genya also joins the fight, mainly as a means to find closure between the Shinazugawa brothers.
All signs point to Zenitsu's destiny in coming face to face with his former superior, Kaigaku, the new Upper Moon 6, who was The Bully to him in his childhood, and has now made it even personal by turning into a demon, resulting in a domino effect that is Jigoro committing seppuku alone. Past history aside, Kaigaku could not have been a better Evil Counterpart to Zenitsu than anyone else, being a fellow Thunder Breathing user representing the absolute worst of what Zenitsu would have become if he let his cowardice overtake him.
Obanai and Mitsuri pitted against the new Upper Moon 4 Nakime makes sense if one thinks about it. One, they don't have much of a connection with the other Upper Moons considering their lack of past history with them, and are less powerful than the other three Hashira pitted against Kokushibo. Two, Obanai and Mitsuri fighting together shows they were always destined to be a couple from the very beginning, Obanai falling for her from the moment they have met, and the two declaring their love for each other in their last moments after the Final Battle against Muzan. Also, Mitsuri against Nakime counts as a Designated Girl Fight.
Muzan seems to have a preference for creating demons of people who are either in despair (such as Akaza, Gyutaro and Daki, Rui etc), plain evil (such as Kaigaku, Gyokko, Doma, Hantengu etc) or some combination thereof (such as Kokushibo). Why? Well, Nezuko and Tamayo prove it is possible for demons to resist their man-eating, predatory nature. Someone who is already evil will likely simply induge their demonic side, while those in despair will either lash out or be too broken to keep their human side intact.
Muzan being so Stupid Evil, even having been around for a thousand years, seems a bit odd. But with the revelation of his backstory, it's likely that his family heavily sheltered and coddled him out of fear he could die if he left their sight, so his sense of right and wrong was left horribly undeveloped.
Not to mention it was predicted that he’d die before he reached the age of 20, indicating that he was at most 19 when he became a demon, so he never fully grew up.
On a similar note, Muzan had been a shortsighted, Tranquil Fury tantrum-throwing, narcissist all his life, impulsively doing whatever he wanted without a care in the world. So it makes sense that his final form before dying is a massive infant.
It's mentioned that Muzan doesn't like Doma, despite both of them being evil demons. However this makes sense considering that Muzan is a guy that demands praise and fear from his minions, two things Doma can't feel for him no matter how much he fakes his emotions.
In the Tanjiro's Status Report one-shot set after the events of the final arc, Zenitsu writes and publishes an autobiography where he exaggerates his heroic feats as a demon slayer, including him stating he became the Thunder Hashira. However, this is technically only a half-lie considering how he killed Kaigaku (the upper-moon who replaced Gyutaro) completely on his own - meaning he did in fact meet the requirements to be given the title before the Demon Slayer Corp folded.
